Reactor for decomposition of ammonium dinitramide-based liquid monopropellants and process for the decomposition

ABSTRACT

The present invention relates to a reactor for the decomposition of ammonium dinitramide-based liquid monopropellants into hot, combustible gases for combustion in a combustion chamber, and more particularly a rocket engine or thruster comprising such reactor and a combustion chamber. The invention also relates to a process for the decompostion of ammonium dinitramide-based liquid monopropellants.

FIELD OF THE INVENTION

The present invention relates to a reactor for the decomposition ofammonium dinitramide-based liquid monopropellants into hot, combustiblegases for combustion in a combustion chamber, and more particularly arocket engine or thruster comprising such reactor and a combustionchamber. The invention also relates to a process for the decompositionof ammonium dinitramide-based liquid monopropellants.

BACKGROUND ART

In space applications, such as rockets, satellites and other spacevehicles, liquid propellant thrusters and rocket engines are often used.Such thrusters and rocket engines can for example be used for thepurpose of positioning and attitude control of satellites and otherspace vehicles. For such purposes attitude control thrusters operatingin the thrust range of typically 0.5–50 N, or Δ-V rocket enginestypically operating in the thrust range of 1 N to several kN. Attitudecontrol thrusters are required to perform short pulses or pulse trains,the duration of which typically can be fractions of seconds to severalminutes.

Liquid propellants can be divided into monopropellants andbipropellants. The former consists of one component, while the latterconsists of two components, i.e. a liquid oxidiser and a liquid fuel.The distinction between monopropellants a bipropellants is madeaccording to the number of components which are injected into the enginefor combustion for the specific propellant.

In the case of a monopropellant, which may be a mixture of severalcompounds or one single chemical, only one component is injected intothe engine.

Currently, hydrazine is about the only liquid monopropellant widely usedfor generation of hot gases. In the case of hydrazine the decompositionpathway occurs in two stages; first hydrazine is catalyticallydecomposed into hydrogen and ammonia in an exothermal reaction, andthereafter ammonia further decomposes into hydrogen and nitrogen in anendothermal reaction due to the high temperature generated in the firststage. The second stage endothermal reaction will reduce the flametemperature and reduce the specific impulse. It is therefore desirableto limit the ammonia dissociation as much as possible. When the ammoniadissociation is held to 55%, the adiabatic reaction temperature will beca. 900° C.

In a bipropellant engine, fuel and oxidizer liquids are injected,atomised and mixed in a first zone of the combustion chamber. In thecase of a hypergolic bipropellant, such as hydrazine and nitrogentetroxide, there is an initial chemical reaction in the liquid phasewhen a droplet of fuel impinges with a droplet of oxidiser.Bipropellants which are not hypergolic use some type of ignitor toinitiate the chemical combustion. In a bipropellant system usinghydrogen peroxide as the oxidiser, a catalyst may be used.

Liquid bipropellants generally offer higher specific impulse than liquidmonopropellants. Bipropellant systems are thus more efficient thanmonopropellant systems, but tend to be more complicated because of theextra hardware components needed to make sure the proper amount of fuelis mixed with the proper amount of oxidiser.

Liquid monopropellants, based on a dinitramide compound, and especiallyammonium dinitramide (ADN), have recently been developed, and aredisclosed in WO0050363. These propellants are novel High PerformanceMonopropellants, which generate extremely high temperatures at propercombustion thereof. Such monopropellant comprises at least twocomponents; a dinitramide compound (oxidiser) and a fuel. An additionalsolvent component may also be included, such as water.

These new monopropellants, including at least two components, have beendescribed to generate a very high temperature on combustion, such asabout 1700° C. As the propellant may also include water, very highdemands will be put on a suitable engine or thruster for such a fuel,consequently excluding all known monopropellant thrusters as suitablealternatives.

Thus, it is an object of the present invention to provide a reactor fordecomposition and combustion of liquid ammonium dinitramide-basedmonopropellants.

It is a further object of the present invention to provide a process fordecomposition ammonium dinitramide-based monopropellants, such as forrocket propulsion and for controlled gas generation for any otherpurpose, such as rotary power in auxiliary power units.

Other objects and advantages of the present invention will becomeevident from the following description, examples, and the attachedclaims.

The terms rocket engine and thruster will be used interchangeably hereinto designate the portion of a liquid propellant rocket engine, in whichthe propellant is injected, extending downstream to the nozzle.

SUMMARY OF THE INVENTION

The present inventors have investigated the decomposition pathway ofammonium dinitramide-based liquid monopropellants and found a pathwaythat corresponds to observed temperatures at different stages ofdecomposition. Consequently, it has been found that the combustion of aammonium dinitramide-based liquid monopropellant can be divided into aseries of steps, including i.a. the decomposition of the ammoniumdinitramide oxidiser which eventually generates free oxygen.

In a final stage, combustible components generated from thermal andcatalytic decomposition will be oxidised in a homogenous gas combustionby the free oxygen thus generated. This combustion requires nocatalysis.

Thus, the ammonium dinitramide-based monopropellant can be regarded asbeing decomposed into a bipropellant, which is combusted in a final stepor steps, during which the maximum temperature is reached.

It has also been found that for the desired decomposition of theammonium dinitramide oxidiser to take place, catalytic activity isrequired until the homogenous gas combustion. In practice, this willmean that catalytic activity will be required up to a temperature of atleast 1000° C. As will be discussed in greater detail below, catalyticactivity, especially in the aft portion of the catalyst, may be requiredmay at higher temperatures, depending on the specific monopropellant.

Thus, a key to this goal is the development of a suitablehigh-temperature catalyst. Such catalyst has now been found by thepresent inventors and will be described in detail below.

The inventors have also developed a reactor for the decomposition ofammonium dinitramide-based liquid monopropellants, and a thrusterincorporating the reactor. Such reactor and thruster will be describedin detail below.

The reactor could also be used for generating hot gases at high pressurefor driving a turbine, vane motor, or piston motor.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

FIG. 1 shows adiabatic temperatures after the first reaction stepsduring decomposition of an ammonium dinitramide-based monopropellantcontaining glycerol as the fuel, and after complete combustion. Theinitial temperature is 20° C. 30% of the AN is assumed to decompose tonitrogen and nitrogen dioxide.

FIG. 2, same as in FIG. 1, but with methanol as the fuel.

FIG. 3A illustrates a reactor, and 3B a rocket engine of the inventioncomprising the inventive reactor, wherein 5 is a hollow body, 10propellant feed pipe, 20 injector, 25 heat bed, 30 catalyst bedcontaining catalyst pellets 35, 40 is a retainer, 50 combustion chamberand 60 is a heat and chemically resistant liner.

FIG. 4 shows the thermal stability of the LHA materials prepared inExamples 1–3 by combined sol-gel and microemulsion techniques arecompared to those of conventionally co-precipitated LHA(co-precipitation of carbonates) and commercial alumina samples.

FIG. 5 illustrates pressure traces from 2 test firings with LMP-103 inan experimental breadboard rocket engine of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While not wishing to be bound to any theory, the following reactionscheme is believed to be descriptive of the general reactions governingthe decomposition of an ammonium dinitramide-based monopropellant.

The conversion of the propellant starts with the decomposition of theoxidising agent, ADN, and is followed by the combustion of the fuelcomponent. The initial step is the decomposition of ADN. ADN undergoesthermal decomposition to ammonium nitrate (AN) and nitrous oxide (1):NH₄N(NO₂)₂→NH₄NO₃+N₂O ΔH=−136 kJ/mol  (1)

The formed AN thermally decomposes either to nitrous oxide and water(2), or to nitrogen, nitrogen dioxide and water (3):NH₄NO₃→N₂O+2H₂O ΔH=−37 kJ/mol  (2)NH₄NO₃→¾N₂+½ NO₂+2H₂O ΔH=−102 kJ/mol  (3)

For thermal decomposition, the formed amount of nitrogen and nitrogendioxide decreases with temperature (26% N₂ at 290° C. and 6% N₂ at 340°C.) [J. C. Oxley, J. L. Smith, W. Zheng, E. Rogers, M. D. Coburn, J.Phys. Chem. A 101 (1997) 5646]. Nitrogen dioxide is a very reactiveoxidiser and reacts with the fuel component, such as for exampleglycerol (generally represented by R—CH₂OH in reaction (4) below):R—CH₂OH+NO₂→R—COOH+½N₂+H₂O ΔH≈−470 kJ/mol  (4)

When all AN and nitrogen dioxide are consumed, the remaining oxidiser isin the form of nitrous oxide. Nitrous oxide is decomposed to nitrogenand oxygen:N₂O→N₂+½ O₂ ΔH=−82 kJ/mol  (5)

The released oxygen reacts to complete combustion of the remainingglycerol and the partially oxidised glycerol. In FIG. 1 the adiabatictemperatures after the first four steps and after complete combustionare shown. The initial temperature is 20° C. The heat released duringADN decomposition to AN is less than the heat required to vaporise thewater and the glycerol. The adiabatic temperature after AN decompositiondepends on the extent the decomposition following reaction 2 or 3. If30% of the AN decomposes to nitrogen and nitrogen dioxide (reaction 3)the adiabatic temperature becomes 147° C. If a larger part of the ANdecomposes via this route the adiabatic temperature will be higher. Whenthe formed nitrogen dioxide reacts with glycerol the adiabatictemperature increases to 371° C. and decomposition of the nitrous oxideraises the adiabatic temperature to 765° C. The adiabatic temperaturefor complete combustion is 1700° C.

The change of fuel component in the monopropellant does not influencethe first three reaction steps. Accordingly, when the fuel is methanol,the fourth step applies with the difference that it is now a specificreaction instead of a generalised reaction, i.e. methanol oxidation toformic acid by NO₂ (as shown in reaction 6 below) instead of alcoholoxidation to carboxylic acid by NO₂. This means that the reaction heatis also specified.CH₃OH+NO₂→HCOOH+½N₂+H₂O ΔH=−461 kJ/mol  (6)

Reaction step five also applies without change. The last steps where thereleased oxygen reacts with the remaining methanol and the formed formicacid may also be specified:CH₃H+O₂→HCOOH+H₂O ΔH=−428 kJ/mol  (7)HCOOH+½O₂→CO₂+H₂O ΔH=−248 kJ/mol  (8)

In FIG. 2 the adiabatic temperatures after each successive step andafter complete combustion are shown. The temperatures after the firststeps are similar to the temperatures obtained for the glycerolcontaining propellant. The small differences are due to difference inthe heat capacity for the fuel component and its reaction product. Theheat released during ADN decomposition to AN is still less than the heatrequired to vaporise the water and the methanol. With reference to FIG.2, and with the same conditions as for the glycerol containingpropellant above, the adiabatic temperature after AN decompositionbecomes 162° C., and when the formed nitrogen dioxide reacts withmethanol the adiabatic temperature increases to 379° C. Decomposition ofthe nitrous oxide raises the adiabatic temperature to 778° C. and aftercomplete oxidation of methanol to formic acid the temperature is 1345°C. The adiabatic temperature for complete combustion is 1725° C.

The above suggested reaction schemes are in well agreement withtemperatures measured at different locations along the reactor of asmall thrusters during operation.

According to the above reaction scheme, the net formula for the overallreaction of the monopropellant composition LMP103 can be described as:94H₂O+33NH₄N(NO₂)₂+22CH₃OH->22CO₂+204H₂O+66N₂

For same composition, minus the water, the corresponding net formula is:3NH₄N(NO₂)₂+2CH₃OH->2CO₂+10H₂O+6N₂

With reference to FIGS. 1 and 2, the combustion stage where no furthercatalysis is required occurs shortly after the N₂O decomposition in therespective diagram, and is in practice believed to occur at 1000–1200°C., depending on the specific propellant.

Reaction Kinetics and Consequences for Reactor Design

The ADN decomposition is thermally activated and consequently takesplace in the heat bed if the temperature is high enough. The activationenergy for ADN in solution has been determined to 155 kJ/mol, and 167kJ/mol for neat AND (J. C. Oxley et al.). Ammonium nitrate (AN)decomposition has a lower activation energy (90 kJ/mol according to S.Vyazovkin, J. S. Clawson, C. A. Wight, Chem. Mat. 13 (2001) 960) andconsequently the ADN decomposition is immediately followed by thedecomposition of AN to H₂O, N₂O, N₂ and NO₂. As no further reactiontakes place before the decomposition of ADN has started, the conditionfor a proper ignition is that this reaction reaches a sufficient rate.This is obtained if the heat bed has a sufficiently high temperature. Analternative is to catalyse the decomposition in the catalyst bed.However, if the system is operated that way a problem may arise whenrunning long pulses. During long pulses the heat bed will be cooled downand eventually not all of the liquid will be evaporated before the fluidenters the catalyst bed When liquid enters the porous catalyst bed itcauses desintegration of the catalyst pellets and hence degradation ofthe catalyst bed. This may be prevented if the complete or the majorpart of the propellant conversion takes place a sufficiently shortdistance into the catalyst bed so that the heat conducted upstream tothe heat bed is sufficient to keep the evaporation process inside theheat bed. However, this requires that the catalyst activity is highenough in the beginning of the catalyst bed and that the activity doesnot degrade significantly with time. Another alternative is to use acatalytic heat bed. If the heat bed is covered with a material activefor ADN decomposition sufficient ADN conversion rate will be reached ata lower temperature.

Feasible catalyst candidates for ADN decomposition contain Cr, Co, Mn,Cu or combinations thereof. These metals are known to catalyse thedecomposition of ammonium nitrate (D. E. Petrakis, A. T. Sdoukos, P. J.Pomonis, Thermoch Acta 196 (1992) 447). It is required that the catalystmaterial is not dissolved when the reaction proceed. As the environmentinside the heat bed can be very aggressive (some of the reactionintermediates are NO₂ and also HNO₃ formed from NO₂ and water), it isreasonable to choose Cr, Mn or a combination. These candidates will stayinsoluble if the oxidation state does not reach the highest ones forthese metals. Due to the aggressive and oxidising environment it mayhowever be necessary to use other candidates or combinations with othercandidates, that are more durable, but not as active for ADNdecomposition. Such candidates are Ir, Pt and Rh.

The Reactor

The reactor of the invention can for example form part of a rocketengine or thruster.

In its most general embodiment and with reference to FIG. 3A, thereactor of the invention comprises a hollow body 5 provided with, fromthe upstream end;

-   an injector 20;-   a heat bed 25; and-   a catalyst bed 30 of porous catalyst pellets 35 which are heat and    sintering resistant to a temperature of at least 1000° C.,    wherein the injector is formed so to be able to distribute the    liquid propellant over the heat bed, the overall void volume is    essentially formed of the porosity of the heat bed and catalyst bed,    and wherein the hollow body is thermally conductive and the heat bed    is in indirect thermal contact with the catalyst bed via the hollow    body.

In the case of a rocket engine, the reactor forms part of the engine asshown in FIG. 3B. For simplicity, any conventionally used parts whichare attached to a rocket engine, such as the upstream parts; propellantpump, propellant valve and thermal standoff, etc, as well as a heaterfor heating the heat bed (as conventionally used for heating thecatalyst bed in the case of a hydrazine engine) and thermal standoff forthe heater, have been excluded from the Figure. The skilled person willimmediately recognise which further parts are required for the rocketengine, having read this disclosure. Accordingly, the hollow bodyconfining the reactor is the hollow body of the engine, into which bodythe propellant is injected and combusted. Thus, there is a combustionchamber downstream of the reactor, for combustion the combustiblecomponents generated by the reactor.

Components of the Reactor

The present inventors have surprisingly found the specific type ofinjector to be of great significance for the proper function of thereactor. Accordingly, the function of the present injector is to merelydistribute the propellant over the heat bed, i.e. no atomisation isrequired or desired. The injector should be free of any undesired voidvolumes in which pools of propellant could accumulate. Such pools couldotherwise cause “hard starts” which are severely detrimental to a rocketengine and its components. The injector can be separate from the heatbed and be positioned placed immediately upstream of the heat bed. Aporous disc injector (for distribution only) has been found to performthe intended function, and is presently preferred. Alternatively, theinjector can also be made integral with the heat bed. In this case, theupstream portion of the heat bed could be formed for effectivelyperforming the function of distributing the propellant over the integraldownstream portion of the heat bed.

The heat bed is provided in order to vaporise the propellant beforeentering into the catalyst bed. The heat bed must exhibit sufficientheat capacity in order to vaporise a sufficient portion of thepropellant being fed into the bed during start and before heat is beingtransferred upstream to the bed, as will be described in more detailbelow. The heat bed must also exhibit a sufficient thermal conductivityin order to be able to dissipate heat throughout the bed, which heatpartly will be transferred from downstream to the bed via the walls.This heat it then transferred to the propellant flowing through the bed.Furthermore, the material of the bed must be able to withstand anydetrimental impact from components generated on decomposition of ADN inthe bed, such as nitric acid. Accordingly, the material of the heatshould be acid resistant, and can for example be in the form of a metalmesh.

The high temperature catalyst pellets used in the catalyst bed of thereactor must exhibit a sufficient surface area for the desired catalyticfunction to be performed. The pressure drop over the catalyst bed mustnot be unduly high, since this would cause pressure oscillations andwear of the engine. It is therefore preferred that the catalyst pelletsare porous. The required surface area of the catalyst should also bemaintained for a sufficiently long period of time of operation of theengine using the catalyst, when the catalyst is exposed to the relevantgaseous species, such as water vapour, at a temperature of at least1000° C. In practise, this means that the catalyst must be sinteringresistant at these temperatures. The catalyst should preferably also bedimensionally stable. It is also preferred that the catalyst maintainsufficient catalytic activity to perform its intended function evenafter exposure to 1200° C., preferably 1400° C., and more preferablyeven after exposure to 1700° C.

In order to obtain the desired sintering resistance, the catalystpellets are thermally stabilised at the expected temperature in anatmosphere of the relevant gaseous species. During this treatment somesurface area of the material will necessarily be lost. Accordingly, itis required that the specific catalyst material used is such that itwill maintain sufficient surface area after this treatment The treatmentwill also prevent any undesired shrinkage of the catalyst pellets, whensubjected up to said expected temperature in the reactor.

With reference to FIGS. 1 and 2, in an ideal catalyst, a heat gradientroughly corresponding to the temperature at AN decomposition to thetemperature at N₂O would be established in the catalyst bed. Thedownstream portion of the catalyst will obviously be expected to besubjected to a higher temperature than the upstream portion, as isevident from FIGS. 1 and 2. Also, variations in monopropellants alsotend to lead to variations in the expected temperature, and especiallythe downstream catalyst temperature, such as from 1000–1200° C.

Especially the downstream portion may also be subjected to even highertemperatures, e.g. due to transient undesired upstream shifting of thegradient, such as up to 1400° C., or even 1700° C.

For these reasons and for the obtaining of margins in operationalperformance, such as the life time of the reactor and engine, it ispreferred that the downstream portion is sintering resistant up to 1200°C., preferably up to 1400° C., more preferably up to 1700° C.

By only stabilising the downstream portion of the catalytic bed to suchhigher temperatures, a larger catalytic surface area can be maintainedin the upstream portion of the bed, where the requirements of catalyticsurface are expected to be higher.

The high temperature resistant catalyst pellets contained in thecatalyst bed are thus suitably formed of a refractory, ceramic,sintering resistant material, and the surface thereof is preferablyprovided with one or more suitable catalytically active metals, such as,Pt, Ru, Pd, Pt/Rh, Ir, Rh, Mn or Ir/Rh, more preferably Pt/Rh, Ir, Rh,or Ir/Rh, of which Pt/Rh presently is preferred. This can for example beobtained by impregnation of the pellets with the catalytic metal(s) bymeans of incipient wetness.

When the downstream portion of the catalytic bed is sintering resistantup to a higher temperature than the upstream bed, as described above,the catalytic metal of the upstream portion can be selected as describedabove, i.e. Pt/Rh is presently preferred, while the downstream portionof the bed preferably is provided with one or more suitablecatalytically active metals, such as, Pt, Ru, Pd, Pt/Rh, Ir, Rh, Mn, orIr/Rh, of which Ir and Mn presently are preferred.

While any known material fulfilling the above-mentioned requirementcould be used as the catalyst, the number of known candidate materialsare very limited. The catalyst support is preferably a sinteringresistant alumina material, although other relatively inert metal orceramic sintering resistant support materials could be used instead of,or in combination with, the alumina. The catalyst material is preferablynano-structured, i.e. the material is in the form of nanoparticles. Morepreferably the catalyst comprises a hexaaluminate material. Suchcatalyst will be described in more detail below.

The catalyst pellets can be formed by any suitable forming technique forforming porous pellets of a sintering resistant material. Sphericalpellets are preferred, and should preferably exhibit an even surface.

As an example, the pellets can be formed of a slurry of the specificsintering resistant material and a consolidating agent. Suitableconsolidating agents can for example be selected from starch (starchconsolidation), protein (protein coagulation), and polymers (gelcasting). The solvent used in the slurry can also function asconsolidating agent, in which case the pellets are cooled forconsolidation by freezing of the solvent. The slurry can then formedinto pellets by any conventional forming technique. Thereafter, thepellets are treated (heated or cooled) for consolidation, and theconsolidating agent is burned-out. Finally the pellets are sintered,preferably without the use of any mould, since any applied pressurecould compromise the desired micro porous structure of the pellets, e.g.an open porous structure of 100–200 nm pore size. Preferably, theburn-out step is preceded by a drying step in order to prevent any rapidbuild-up of pressure due to vapours generated, which could causemechanical damage to the pellets.

For the preparation of smooth spherical pellets of a diameter of from0.2–5 mm the below method is preferred. Preferably, these pellets aremade of a hexaaluminte material, as will be described below.

The catalyst bed is thus dimensioned so as to provide the requiredoverall catalytic surface area, and such that the final combustion stageessentially will occur downstream the catalyst bed, i.e. in thecombustion chamber, while at the same only offering an acceptablepressure drop across its length in the direction of the flow.

The catalyst bed is kept in place by a retainer. An example of asuitable retainer is a perforated plate of Ir or Ir supported by Re, asIr is inert to the relevant combustion species.

Combustion chamber (first void volume of reactor). The walls of thereactor, including the combustion chamber, must be able to withstand thehigh temperatures generated during combustion of the propellant. Theymust also be resistant to any exhaust gases or intermediarydecomposition products generated in the reactor. A suitable material isthus rhenium. In order to withstand the nitric gases generated in thefinal steps of the decomposition the combustion chamber portion of thewalls are suitably lined with iridium.

The desired characteristic chamber length, L*, for ADN based liquidmonopropellants is estimated to be approximately 0.5 to 2 m.

Any voids in the reactor chamber should be kept to a minimum in order toavoid hard starts, and undesired damage of reactor, primarily due topotential accumulation of liquid propellant in such voids. The onlydesired void volume is the combustion chamber. The remaining void volumeof the reactor should be comprised of the pores and interstitial voidsin the catalyst bed and voids in the heat bed.

Accordingly, it is preferred that the catalyst material be heatstabilised at the expected maximal catalyst temperature, before use inthe catalyst bed. By doing so, any reduction of the catalyst bed volumeduring operation of the reactor is reduced to a minimum. Morepreferably, such stabilisation is performed at a temperature 50° C.above the expected maximum catalyst temperature.

Function of the Reactor and Process of Decomposition

The function of the reactor and its components will now be described inmore detail with reference to a rocket engine (see FIG. 3B)incorporating the reactor.

Before starting the reactor the heat bed is pre-heated to a sufficienttemperature presently demonstrated to be 280° C. and above. The reactorwill start already with a heat bed temperature of 250° C., but in orderto obtain a nominal start 280° C. is preferred. The power of the heatingis usually only enough to heat the bed to a sufficient temperaturewithin a reasonable time, such as 10 to 30 minutes. In the case of asmall thruster of about 1 to 10 N, the power is typically less than 10W. The fuel, usually at a temperature of 10–50° C., is then pressure fedinto the heat bed. As the propellant enters the heat bed it propagatesdownstream through the pre-heated bed. The propellant fed into the bedwill cool the bed down during its passage. The temperature and the heataccumulated in the bed will however be sufficient to vaporise a firstportion of propellant fed into the bed. The vaporised portion will befollowed by a boiling front moving down-stream through the bed as thetemperature of the bed further downstream decreases. Without any furtherheating the bed would eventually reach the temperature of the propellantthat is being fed into it. At the same time, and with reference to thereaction scheme outlined above, the heat from the heat bed will initiatedecomposition of ADN contained in the monopropellant. As previouslymentioned, although this reaction is exothermal, it is not believed tobe enough itself to vaporise the propellant.

As the vaporised portion of the propellant enters the catalyst bed, aseries of reaction step will follow, finally producing free oxygen thatwill oxidise the remaining fuel component and any combustible componentsproduced in previous steps. The final step or steps comprising thecombustion stage take place in the combustion chamber. The overallreaction taking place in the catalyst bed and combustion chamber ishighly exothermal.

In order to avoid mechanical failure of the porous catalyst bodies dueto evaporation induced disintegration thereof, it is crucial that enoughheat is transferred upstream to the heat bed for the heat bed to be ableto vaporise the fuel fed into the bed at such rate that the boilingfront does not reach into the catalyst bed.

Preferably, also the dual phase flow consisting of liquid particles invapour, between the moving boiling front (vapour in liquid) and themono-phase gas flow, should not reach into the catalyst bed. Preferably,the catalyst bed should be dimensioned accordingly.

As the decomposition reactions progress in the catalyst bed andcombustion chamber, the catalyst bed will obtain an increasingly highertemperature downstream. During combustion in the combustion chamber,heat will inevitably partly be lost to the surrounding walls. Heat fromthe catalyst bed will also partly be lost to the walls. A part of thisheat is conducted upstream via the walls to the heat bed, which willreceive and dissipate the heat throughout the heat bed. The heat bedwill in turn transfer heat to the propellant being fed into the bed.Some heat will also be radiated from the catalyst bed and received bythe heat bed.

Thus, the boiling front of the propellant being fed into the heat bedwill initially advance downstream in the heat bed, and once the heatgenerating reactions are initiated in the catalyst bed and combustionzone, heat will be transferred upstream causing the boiling front tostop or be shifted upstream in the heat bed. After these starttransients, the boiling front will reach a fairly stationary positionwithin the heat bed, where heat transferred to the bed corresponds tothe heat lost from the bed to the propellant passing through the bed.

In summary, the heat bed will vaporise the propellant, and, at the sametime, initiate the decomposition of ADN, which, according to theproposed reaction scheme, is necessary in order to initiate thecombustion of the fuel.

As previously mentioned, by including catalytic activity in the heat bedfor the purpose of enhanced decomposition of ADN, it might be possibleto reduce the required pre-heating temperature.

In the catalyst bed the temperature will increase in the downstreamdirection. As already mentioned, the bed should be dimensioned so thatthe combustion stage occurs in the combustion chamber. Should combustionoccur in the catalyst bed due to any undesired unstable conditions, suchas for example, during a cold or hard start, combustion may transientlybe shifted into the aft portion of the catalyst bed, with theconcomitant exposure of catalyst pellets in said aft portion totemperatures severely higher than the intended optimal operationaltemperature in the portion of the bed, such as about 1400° C. or even1700° C.

Such exposure will decrease the catalytic activity of the pellets thusexposed. Mainly because of loss of the impregnated catalytically activemetal from the surface of the pellets.

It has been found that by including a suitable dopant, such Mn, in thecatalyst material, catalytic activity will be maintained as long as thecatalytic material itself is not decomposed.

However, in the upstream end of the catalyst bed a reasonably highcatalytic surface area is required for the reactions to take place at asufficiently high rate. Accordingly, combustion should be prevented fromoccurring to far upstream into the catalyst bed, or catalytic activitymight be lost, shifting the combustion to an undesirably aft position,or even leading to wash out of the engine during either of the followingpulses.

With a suitable dopant in the catalyst material, and especially of thecatalyst pellets have been dimensionally stabilised at 1400° C., thepresent inventors believe transiently occurring combustion temperaturesin the aft 25% of the bed not to cause any serious damage to the engine.

Preferred Catalyst Material for the Reactor

The present inventors have found that most known catalyst supportmaterials, when exposed to the thermal and chemical conditions generatedon combustion of the ammonium dinitramide-based monopropellantsdisclosed in WO-0050363, will sinter at a rate which is too high for thematerial to be of practical value in a catalyst bed of the inventiverocket engine. By the sintering of the catalyst support material, thecatalytic surface will be severely reduced, and the catalyst willfinally not be able to perform its function any longer, and thus, therocket engine will not operate properly, if at all. In order to reliablyperform its function, it is important that the catalyst retain itsintended surface area for a sufficiently long period of time ofoperation of the engine containing the inventive catalyst.

In a preferred embodiment the refractory, ceramic, sintering resistantmaterial used for preparing the catalyst pellets is a ceramic powderprepared by the following method.

In its most general concept, the method is based on amicroemulsion-assisted sol-gel technique, according to which methodhexaaluminates, AA1 ₁₁O₁₈, wherein A is an alkaline earth or rare earthmetal, having improved sintering resistance are prepared by adding asolution of an aluminium alkoxide to a water-in-oil microemulsion, theaqueous phase of which comprises a solution of a water soluble salt ofA, whereafter the powder formed is recovered and calcined.

Accordingly, in this method, at least one of the components forming thepowder is included in the water phase of the micro-emulsion.

It has been found that by including the alkaline earth or rare earthmetal in the water phase of the micro-emulsion, improved compositionalhomogeneity of the powder formed is obtained, thus enablingcrystallisation at a lower temperature, suppressing grain growth andleading to a reduced loss of surface area during crystallisation, andthereby a hexaaluminate powder having larger specific surface area.

Furthermore, by means of the inventive method the nano-particlesobtained exhibits a very narrow particle size distribution, which isbelieved to further enhance the obtainable specific surface area of thehexaaluminate.

Since sintering is suppressed, the material will thus exhibit enhancedthermal stability. The surface area of the powder is thus maintainedbetter at high temperatures, even in the presence of water vapour.

The present method is offers a less expensive route to hexaalummates,since the water soluble metal salts, and especially the nitrates,generally are much cheaper than the corresponding alkoxides.

The method is also simplified since alkoxides are generally moredifficult to handle. Furthermore, the alkoxides will generally have tobe added to an organic solvent, since they are not water soluble, suchfor example the iso-propoxides.

In order to obtain an improved catalytic activity at extremely hightemperatures, a portion of the aluminium alkoxide can be substituted byan equimolar amount of a water soluble salt of manganese. The manganesesalt is then added to the water phase, together with the water solublesalt of the alkaline earth or rare earth metal.

The method can be used with any water soluble salt, such as chlorides oracetates.

Nitrates are suitably used in the method since the nitrate moiety iseasy to strip from the precipitated powder, and are also generallyreadily available.

The use of a nitrate metal salt in the method also simplifies thesubstitution of manganese for lanthanum, since manganese nitrate is morereadily available, than manganese alkoxides.

The metal salts used should preferably exhibit the same anion.

It is preferred that the solvent for the aluminium alkoxide and thesolvent of the oil phase of the microemulsion to the same. Morepreferably, a solvent is selected which can readily be evaporated forrecovery of the powder.

The sol-gel technique is well-developed for the preparation ofhigh-surface area metal oxides. By combining the sol-gel technique withmicroemulsion-mediated synthesis, we have developed a method, whichenables preparation of a nanostructured hexaaluminate material, andespecially LHA, with high-temperature stability and enhanced resistanceto sintering compared to hexaaluminate prepared by conventionaltechniques. A water-in-oil (w/o) microemulsion, sometimes referred to asa reverse microemulsion, contains well-dispersed and nanometer-sizedwater droplets of a narrow size distribution. The water-oil interface isstabilised by amphiphilic molecules (surfactant molecules). By using thedroplets as nanoreactors, conventional water-based chemical reactionscan be carried out in a well-defined and confined environment. This isknown as microemulsion technique.

By using the microemulsion technique, the composition of the precipitateis controlled not only by the rates of precipitation, but also by thediffusion rate of each reacting component in the solvent phase. Thisfeature, combined with the confined environment provided by thenanodroplets, enables synthesis of nanostructured materials. It isbelieved that the formation of hexaaluminate crystal structure at lowertemperatures is favoured if the precipitate is well-mixed at nanometerlevel. Nanoparticles generally exhibit high reactivity, due to the highsurface/volume ratio. Once the hexaaluminate phase has formed, furthercrystallite growth is slow, thereby leading to good high-temperaturestability. In contrast, when conventional preparation techniques areused, higher temperatures are generally needed for crystallisation,leading to sintering of the material accompanied by loss of surfacearea.

The hexaaluminate is prepared by hydrolysis of an aluminum iso-propoxidesolution, using a microemulsion containing, for example, metal nitratesin the aqueous phase. The gel is aged under stirring, during whichhydrolysis and condensation occurs. Subsequently, the powder isrecovered, dried and calcined.

In the examples below, lanthanum hexaaluminate powder (LHA, LaA₁₁O₁₈) isprepared. More specifically, the method involves the hydrolysis of analuminium alkoxide, preferably aluminium iso-propoxide, using an aqueoussolution of a water solule lanthanum salt, preferably lanthanum nitrate,added in the form of a water-in-oil microemulsion.

EXAMPLES 1–3

Powder Preparation

Two different solutions were prepared. Table 1 lists the chemicals thatwere used.

TABLE 1 Substance Chemical formula Purity Manufacturer LanthanumLa(N0₃)₃ * 6 H₂0   99.99 Rhône-Poulenc nitrate (hydrated) AluminiumAl(OC₃H₇)₃ >98 Alfa iso- propoxide Nonylphenol C₇H₁₉C₆H₄(OCH₂CH₂)₆OHIndustrial Akzo Nobel ethoxylate grade Surface (NP-5; trade Chemistryname: Berol 02) Cyclohexane C₆H₁₂ >99 J. T. Baker Distilled H₂0 — —water

The first solution consisted of approximately 15 wt % aluminiumiso-propoxide (Al(OC₃H₇)₃) dissolved in cyclohexane (solution 1). Thedissolution may be aided by ultrasonic treatment.

The second solution was a w/o microemulsion (solution 2) prepared fromtwo different solutions.

First, a solution of lanthanum nitrate (La(NO₃)₃*6H₂O) in distilledwater was prepared. In order to obtain the hexaaluminate phase it isimportant that the molar La/Al ratio is exactly 1:11. The stoichiometricwater/—OC₃H₇ molar ratio is 0.5, i.e. the corresponding water/aluminiumiso-propoxide ratio is 1.5. In the examples 1, 2 and 3, thewater/aluminium iso-propoxide ratio was 10, 50 and 100, respectively,times the stoichiometrically required amount. Hence, the concentrationof lanthanum nitrate in the aqueous solution used in the differentexamples varied depending on the specific water/alkoxide ratio used.

A solution of 20 wt % NP-5 in cyclohexane was then prepared. By addingthe aqueous solution of lanthanum nitrate to the surfactant-solventsolution, a microemulsion was obtained. The amount of aqueous phase inthe microemulsion was always kept at 10 wt %.

Precipitation was accomplished by slowly adding the solution ofaluminium iso-propoxide in cyclohexane (solution 1) to the microemulsion(solution 2) under stirring. The mixture was aged under stirring for 48h, during which hydrolysis and condensation took place. The ageing timemay be increased or decreased.

Thereafter the precipitate was recovered by careful evaporation of thesolvent in an oven at 75° C. in air. The boiling point of cyclohexane is81° C. and this temperature must not be exceeded, as the solvent willstart boiling violently.

Then the powder was calcined in air in a furnace. The temperature wasincreased at 2–5° C./min. The final temperature was chosen between 800and 1200° C. and kept isothermal for 4 h.

The obtained calcined powders were characterised by X-ray diffraction(XRD) to determine the crystalline structure. Nitrogenadsorption-desorption at liquid nitrogen temperature according to theBET method was used to determine the specific surface area of thepowders. The results are shown in Table 2.

TABLE 2 BET surface areas and crystal phases of prepared powders.(calcination: 1200° C., 4 h; hydrolysis: 48 h; surfactant system:NP5/cyclohexane) Water/alkoxide Predominant Minority Ex. ratio (timesBET surface crystal crystal No stoich.) area (m²/g) phase* phase(s). 110 32.9 LHA LaAl0₃ 2 50 35.1 LHA LaAl0₃ 3 100  23.0 LHA LaAl0₃ *LHA =Lanthanum hexaaluminate

EXAMPLE 4

The thermal stability of the LHA materials prepared by combined sol-geland microemulsion techniques was tested and compared to those ofconventionally co-precipitated LHA (co-precipitation of carbonates) andcommercial alumina samples. The tests were carried out under extremeconditions, i.e. 1400° C. and 60% steam, to simulate conditions similarto those prevailing in a rocket engine. The surface areas were measuredby BET and crystal phases were determined by XRD. The results are shownin FIG. 4.

It can be seen that most of the surface area is generally lost withinthe first few minutes. After 15 minutes, the decrease is much lessdramatic. This is probably due to phase transitions in the materials, aswell as rapid sintering of the smaller pores. After about 1 hour, thesurface area is almost constant for up to 10 hours. The lanthanumhexaaluminate (LHA) sample prepared according to the invention exhibiteda surface area of 19 m²/g after 30 minutes. This should be compared to11 m²/g of the LHA catalyst prepared by conventional carbonateco-precipitation and 3 m²/g of the commercial alumina sample.

The choice of surfactant-solvent system greatly influences the dropletsize in the microemulsion and the water solubilisation capacity. Thesurfactant may be ionic or non-ionic, contain branched or straighthydrocarbon chains etc. The solvent is generally chosen to match thehydrophobic tails of the surfactant molecules. We chose to work withNP-5/cyclohexane systems. NP-5 is a non-ionic surfactant containing fiveoxy-ethylene groups in the hydrophilic head group and a nonylphenylgroup as the hydrophobic tail. Although the aluminium iso-propoxide isreadily dissolved in other solvents, it is important to choose one,which is compatible with the microemulsion (solution 2). Hence,cyclohexane was used in the examples. AOT/isooctane could also be used.However, the AOT molecule has two branched hydrophobic tails and hence,this system has a significantly lower water solubilisation capacity dueto the bulky tail group. In addition, there were indications that AOT ismore difficult to remove for powder recovery. AOT is a solid at ambienttemperatures, while NP-5 is in the liquid state. It should be noted thata large variety of different surfactant-solvent systems might be used.

In the above examples the amount of aqueous phase in the microemulsionwas always kept at 10 wt % but may be decreased in order to obtainsmaller water droplets.

The water/surfactant ratio in the microemulsion determines the waterdroplet size and affects the final particle size of the precipitate.Although small water droplets are generally desired, the amounts ofsurfactant and solvent needed increase drastically when reducing thedroplet size, e.g. by a factor ten when reducing the aqueous contentfrom 10 to 1 wt % in a system with constant surfactant/solvent ratio.The composition of the microemulsion is also limited by thecompositional region in which the w/o microemulsion phase is stable. Asis well known to the person skilled in the art of w/o microemulsionseach system has its individual ternary phase diagram, which must betaken into account.

The water/alkoxide ratio affects the nucleation process and the size ofthe precipitated particles. The stoichiometric ratio of water/aluminiumiso-propoxide is 1.5, but ratios in excess of stoichiometry aregenerally used, as rapid precipitation, i.e. small particles, isdesirable.

The ageing time will also affect the properties of the precipitatedparticles.

It is important to maintain the unique, discrete properties of theparticles upon recovery. The final powder morphology is very much aresult of the recovery step. There are several possible methods forrecovery of the precipitate, which could be used, such as filtration,centrifugation, temperature induced phase separation, chemicaldestabilisation, evaporation of the solvent, supercritical drying andfreeze drying. Although conceivable, centrifugation and filtration arenot believed to be of any practical value, due to the extremely smallparticle size. The method of highest practical value is presentlyconsidered to be evaporation of the solvent.

Parameters such as atmosphere, heating rate, final temperature andduration of the calcination treatment all influence the properties ofthe final product.

Preferred Method of Preparing Pellets of 0.2–5 mm in Diameter for theCatalyst Bed of the Reactor.

By means of the following inventive method spherical pellets of 0.2–5 mmcan be prepared from a catalyst material having a high sinteringresistance, such as the hexaaluminate material described above, and/orcontaining a sintering inhibiting agent, without the use of anyconventional mould.

According to the method spherical pellets are prepared from a slurrycomprising a catalyst material, a solvent, and any desired additives, bymeans of a drop-generating orifice to which said slurry is fed. In themethod, the thus-formed drops are released from said orifice by means ofa relative flow of a liquid medium, and formed into spherical bodies insaid liquid medium by means of the action of surface tension, andthereafter treated for consolidation by a suitable direct castingmethod. Preferably, the slurry also contains a consolidating agent. Theconsolidated drops are dried, and thereafter any consolidating agentand/or organic fillers contained in the slurry are burned-out, and thebodies sintered.

By using a catalyst material having a high sintering resistance and/orcontaining a sintering inhibiting agent, pellets having a sub-micronrange porosity can be obtained, which pellets maintain the desired area,or only slowly degrade in their desired area at a high temperature. Thepellets thus formed will also maintain their geometry when subjected tohigh temperatures, also when exposed to relevant fluids, under suchtemperatures.

The diameter of the pellets can be closely controlled by regulating therelative flow rate of the liquid medium, and the feed pressure of theslurry. By means of the method spherical pellets of a controllable,uniform size can be prepared.

As examples of desired additives which can be used in the slurry, thefollowing can be mentioned: dispersants, defoamers, binders, fillers,consolidating agents, and processing aids etc.

When a particulate organic filler, such as in the form of fibres orparticles, and/or a particulate consolidating agent is used in theslurry, the pellets prepared according to the present method willgenerally exhibit some residual porosity from the burnout of the fillerand/or consolidating agent used.

It is also conceivable to use a fibrous or particulate filler,optionally in addition to a consolidating agent, which filler can beremoved by means of burn-out, in order to create pores in the pellets,corresponding to the geometrical shape of the filler after burn-outthereof.

Accordingly, if desired, the amount of consolidating agent can beincreased above the amount necessary for consolidation, in order toobtain open porosity in the pellets after burn-out of the consolidatingagent. A filler which can be burned-out can also be used for the samepurpose. The size of the pores can then be regulated by means of theparticle size of the consolidating agent and/or filler.

In a preferred embodiment starch is used as consolidating agent.

According to the present method, a relative flow of a liquid mediummeans that slurry enters into a flow of liquid medium, or into astationary liquid medium, in which case the opening described a movementrelative to the liquid medium is moved back and forth, or in a circle,for example, relative to the stationary medium.

The direction of the relative flow of the liquid medium is not criticaland can vary from being coincidental with direction of formation of thedrops, to essentially perpendicular to the direction of formation of thedrops, the former of which is presently being preferred

In order to establish the action of surface tension, which is believedto be the principal driving force underlying the forcing of the releaseddrops to assume a spherical shape, a liquid medium which is a poorsolvent for the solvent of the slurry is preferably selected. Thisdesired effect will be enhanced by selecting a medium which isimmiscible with the solvent of the slurry. In any case, the liquidmedium should be effective to force the released drops to minimise theirsurface area.

The spherical drops thus formed are then treated for consolidation, inaccordance with the specific consolidation method, drying, burn-out ofany filler and/or consolidating agent used, and sintering.

By performing the sintering step under pressure-less conditions, i.e.without the use of a mould, the pellets will exhibit a macro-porositydepending on the specific consolidating agent used and/or filler, andmore particularly the particle size and shape thereof.

For a given slurry and given diameter of the opening, the size of thepellets can be closely controlled by regulating the relative flow rateof the liquid medium, and the feed pressure of the slurry. Other factorsthat will affect the pellet sizes obtainable are primarily the viscosityof the slurry, the density of the slurry, and the diameter of theopening.

Any suitable consolidating agent can be used in the present method. Theconsolidating agent will of course be dependent on the desired method ofconsolidation. Suitable consolidating agents and consolidation methods,respectively, are;

-   starch—starch consolidation,-   protein—protein coagulation,-   polymer—gel casting (from monomers, or polymers which are    cross-linked, and-   solvent of the slurry—freezing.

The term direct casting as used in the present application is generallydefined as the process of transforming an amount of slurry into a rigidbody, and is intended to embrace such methods wherein a consolidatingagent is used. The terms direct casting and consolidation will be usedinterchangeably.

Suitably examples of direct casting method are described by Wolfgang M.Sigmund et al in “Novel Powder-Processing Methods for AdvancedCeramics”, J Am Ceram Soc, 83 [7] 1557–74 (2000), which is incorporatedby reference herein in its entirety

In a preferred embodiment of the inventive method, starch is used asconsolidating agent.

For the purpose of regulating the size of the macro pores resulting fromburn-out in the pellets, starch is a very suitable, and can also performthe function of a consolidating agent. The average size of the starchparticles generally ranges from 2–100 μm depending on from which plantthe starch is derived.

Thus, for example, the consolidating amount of starch could be of onesize, and additional starch particles added in order to obtain an openporosity could be of another size. It is also conceivable that suitablestarch addition will reduce the density of the obtained pellets withoutaccomplishing an open continuous porous structure

The design of the apparatus used to form the drops or droplets is riotcritical, and can be of any design as long as drops can be produced.

It is preferred that the drops formed be treated for consolidation aspossible after having attained spherical shape. This can be done bydischarging the drops from the apparatus directly into a suitable mediumfor consolidation. In for example the case of polymers, starch andprotein requiring heating for consolidation, the medium, such as aliquid, is heated to consolidation temperature. Other means of heatingthe drops are of course also possible, such as a heated gas medium ormicrowave radiation. In order to obtain spherical droplets, the dropletsmust have enough time to become spherical, by the force of surfacetension, before the solidification temperature is reached in thedroplets, as this will lock the current geometry.

In the case of consolidation by means of freezing of the solvent of theslurry, a cold medium is used instead.

In the method of the invention, an apparatus according to the followingcan be used, for example. A suitable apparatus in its most simpleembodiment can be based on the following general components. An opening(i.e. drop-generating orifice), such as the opening of a small tube orcannula, from which opening drops are released or ejected, and to whichcannula a slurry to be consolidated is fed. The drops are then forced toseparate from the opening, by means of the flow of a liquid mediumacting on the slurry ejected from the opening. A suitable flow can forexample be achieved by means of a cannula, displaced in a tube, in whichtube the liquid medium flows. The drop formed at the opening of thecannula will the be entailed by the liquid flow and attain its sphericalshape. The flow of liquid medium with entailed drops can for example bedischarged into a tank containing the liquid. Depending on the specificmethod of consolidation, said tank can for example be heated in orderfor consolidation of to take place. Liquid can then be recirculated fromthe tank to the tube, optionally after cooling thereof.

After consolidation, the pellets are preferably dried before burn-out ofany consolidating agent and/or filler, in order to preventdisintegration of the pellets during the burn-out, due to rapid build upof any vapour inside the bodies.

It is generally desirable that the catalyst pellets exhibit an as greatas possible specific surface area, in order to maximise the catalyticsurface area. In this case it is of a great advantage to use a ceramicmaterial having a high resistance to sintering at high temperatures,such as the above-mentioned hexaaluminate. Preferably, such powder has anarrow particle size distribution. Thereby, pellets having a fine(sub-micron range) porosity, such as in the range of 100–200 nm, can beobtained. By using a higher amount of consolidating agent in the slurrythan necessary for consolidation, an open macro-porous structure can beobtained, formed by the pores resulting from burned-out particles ofconsolidating agent and/or filler in the pellets. Thereby, an increasedfraction of the nano-porous structure will be available to catalysis,and thus the pellets will exhibit a substantially increased effectivecatalytic surface area.

Such an open porosity will also reduce the flow resistance posed by thepellets, when contained in a catalytic bed, for example. Also, the riskfor vapour induced disintegration of the pellets could be reduced, sinceany vapour formed in the pores by liquid that has penetrated into thepellet more easily can escape from the structure by means of an openporosity.

By means of varying the amount of consolidating agent (or filler whichcan be burned-out), and thereby the extent of the open porosity, pelletscan be prepared offering a controlled pressure resistance, whencontained in a catalyst bed for example.

According to a preferred embodiment of the method of the presentinvention, drops are formed from a slurry containing a suitable ceramiccatalyst powder, starch as the consolidating agent, optionally adispersant, and water, which drops thereafter are heated for swelling ofthe starch, such as, for example, by being heated in a liquid medium.The slurry can also contain other organic constituents and solvents ordispersing media or liquids, as long as an amount of water sufficientfor effecting swelling is present. Naturally, a liquid medium for theforming of the drops must be selected that does not disturb the functionof the constituents of the slurry.

During heating to elevated temperatures, the starch pellets will absorbwater from the slurry and swell, thereby forming rigid bodies, which canbe collected and dried. During the swelling, the consolidated bodies arepreferably allowed to consolidate (solidify) freely, i.e. without theuse of a mould. The dried bodies are thereafter heated at highertemperatures in order to remove the starch through a burn-out, andfinally sintered at even higher temperatures to achieve a material withsufficient strength and hardness. The macro-porosity remaining in thematerial after sintering will generally correspond to the amount andtype of starch pellets used in the slurry, and the ability of theceramic matrix to densify, the latter of which generates themicro-porisity.

The shape, size and swelling temperature of the starch granules dependson the specific starch type. Among the most common starches forcommercial uses, potato starch swell at 50–55° C., corn and rice starchat 60–75° C. Examples of other varieties of starch which can be used inthe invention are those obtained from the seeds of cereal grains, suchas sorghum and wheat, also from certain roots, such as tapioca, cassavaand arrowroot, and from the pitch of the sago palm. The mean granulesize is 55 μm for potato starch, 10–15 μm for corn starch and 5 μm forrice starch. The size of the starch used is not critical and can beselected based on the specific purpose and the desired size of thepores. The starch can be in native form or in chemical modified form.For example, the starch can be modified by etherification to make itmore stable towards mechanical treatment and acidic conditions.

The present pellet forming method will now be described in more detailwith reference to the following examples.

EXAMPLE 5

Preparation of spheres from a slurry containing an amount of starcheffective for consolidation of the drops.

The constituents used are listed below:

Constituent Designation/Manufacturer Percentage Aluminium oxideAKP30/Sumitomo, 35 vol % (solids powder Japan content) DispersantDuramax D-3021/Rohm and 1.0% by weight Haas France S.A., France based onpowder Liquid Distilled water Balance Starch Mikrolys 54, 1.43 g/cm³/ 5%by vol. based Lyckeby Stärkelse AB, Sweden on powder

The ceramic powder used in the example was aluminium oxide. The oxidewas first dispersed in water together with the dispersant by ballmilling. Then the starch was added during mixing by means of apropeller. In this case a chemical modified and size-fractionated potatostarch with a mean pellet size of 20 μm was used. Thereafter the slurryobtained was forced into a cannula with an inner diameter of 0.3 mm,which was inserted into a polyethylene tube with an inner diameter of3.5. The liquid heating medium was circulating in the polyethylene tubeand the flow forced the drops to be released (at a premature stage) fromthe opening of the cannula. By changing merely the flow velocity of theliquid heating medium, the size of the drops could readily be variedbetween 0.5 and 1.5 mm. The liquid heating medium used, in which thespheres are consolidated, was liquid paraffin (KeboLab, item No.13647-5), and was kept at an elevated temperature of 60–70° C.

The consolidated pellets were collected and dried in air at about 50° C.Thereafter the spheres were burned out at 500° C. and sintered at 1600°C., for 30 minutes in air. The heating ramps used were 1° C./min up to500° C. and 5° C./min up to the sintering temperature.

EXAMPLE 6

Preparation of spheres from a slurry containing an amount of starcheffective for consolidation of the drops.

The constituents used in the example are listed below:

Constituent Designation/Manufacturer Percentage Hexaaluminate LaAl₁₁O₁₈,from emulsion/ 30 vol % (solids powder Kemisk Teknologi, content) KTH,Sweden Dispersant Duramax D-3021/Rohm and 1.0% by weight Haas FranceS.A., France based on powder Liquid Destillerat vatten Balance StarchMikrolys 54, 1.43 g/cm³/ 5% by vol. based Lyckeby Stärkelse AB, Swedenon powder

The ceramic powder used in the example was lanthanum-hexaaluminateprepared in Examples 1–3.

The calcined powder was amorphous and had a very fine particle size andexhibited a specific surface area of 280 m²/g. However, using such afine powder, slurries of sufficiently high solids content are difficultto reach. Therefore, the powder was additionally calcinated at 1200° C.during 4 hours in air. At this temperature the powder is transformedinto a crystalline phase and the specific surface area is reduced to30–35 m²/g.

A slurry was prepared based on the constituents enumerated above.Thereafter the slurry obtained was forced into a cannula with an innerdiameter of 0.3 mm, which was inserted into a polyethylene tube with aninner diameter of 3.5 mm. The liquid heating medium was circulating inthe polyethylene tube and the flow forced the drops to be released (at apremature stage) from the opening of the cannula. By merely changing theflow velocity of the liquid heating medium, the size of the drops couldreadily be varied between 0.5 and 1.5 mm. The liquid heating mediumused, in which the spheres are consolidated, was liquid paraffin(KeboLab, item No. 13647-5), and was kept at an elevated temperature of60–70° C.

The consolidated pellets were collected and dried in air at about 50° C.There after the spheres were sintered at 1200, 1300 and 1400° C.,respectively, for 30 minutes in air. The heating ramps used were 1°C./min up to 500° C. and 5° C./min up to the sintering temperatures.

The pellets obtained after sintering were spherical, and exhibited avery smooth surface, and a high side crush strength. The porosity wasfound to be binomial, with the larger pores resulting from theconsolidating agent particles, and the finer porous structure, 100–200nm, resulting from the specific ceramic powder used. The pellets werefound to be resistant up to temperature of at least 1700° C.

In examples 7 and 8, different catalytic metals impregnated by incipientwetness on the pellets prepared in Example 6 were tested in anexperimental 1 N class rocket engine, according to the presentinvention.

EXAMPLE 7

Iridium on Lanthanum Hexaaluminate Catalyst

The rocket engine was test-fired with a catalyst consisting of 3.2%iridium deposited on lanthanum hexaaluminate pellets prepared in Example6. The reactor was preheated to 300° C. The engine was test-fired with apropellant blend consisting of about 64.3% ammonium dinitramide, about24.3%-water and about 11.4% by weight of methanol, with and withoutstabiliser (0.5% urea). The combustion temperature was measured with athermocouple. A nearly adiabatic exothermic reaction temperature wasmeasured at each firing.

EXAMPLE 8

Platinum/Rhodium on Lanthanum Hexaaluminate Catalyst

An experimental 1 N class rocket engine, built according to the presentinvention was test-fired with a catalyst consisting of 9.4%platinum/Rhodium deposited on a high-surface area lanthanumhexaaluminate pellets prepared in Example 6. The reactor was preheatedto 300° C. The engine was test-fired with a propellant blend consistingof about 64.3% ammonium dinitramide, about 24.3% water and about 11.4%by weight of methanol (in the following called LMP-103). The combustiontemperature was measured with a thermocouple. The combustion temperaturewas measured with a thermocouple. A near adiabatic exothermic reactiontemperature was measured at each firing.

EXAMPLE 9

In this example two test firings using LMP-103 in a 1 Newtonexperimental breadboard rocket engine of the present invention wereperformed. FIG. 5 shows the pressure traces from the propellant feedsystem and the rocket engine combustion chamber, during the two testfirings of 0.3 sec duration each under vacuum conditions. The figurealso demonstrates three essential characteristics; Rapid ignition,Stable combustion, and Blow-down capability.

A number of experimental rocket engines according to the invention havebeen built and tested by firing of more than 100 pulses with durationsbetween 0.1 and 50 s. In addition to the functions described above,complete combustion, and thus high specific impulse, and sustainedreactor functionality after numerous thermal cycles have also beenachieved.

Suitable examples of ADN-based monopropellants which can be used in theprocess of the invention are those described in WO-0050363.

Preferred Monopropellants for Decomposition in the Reactor

Preferred examples of ADN-based monopropellants, which can be used inthe process of this invention, are the above-mentioned propellants, towhich a base has also has been added as a combustion stabilising agent.

By addition of the stabilised combustion characteristics are obtained.An improved storage life, i.e. stability to storage under elevatedtemperatures, is also achieved by means the addition of the baseaccording to the propellant.

The base used as a combustion stabilising agent must be a base weakerthan ammonia, or a base that is sterically hindered. This is in order toprevent the NH₄ ₊ cation from escaping from the propellant in the formof ammonia, thereby leaving the dinitramide anion balanced by othercation species. As a consequence, the solubility characteristics of anyconstituents could possibly also be altered, and there could be a riskof any undesired precipitation. Examples of such bases are: hydrazine,hydroxylamine, urea, ethyleneimine, allantoin, pyridine, 2-, 3-, and4-methylpyridine, 2- and 4-pyridineamine, 2,5-pyridinediamine, 2,3-,2,4-, 2,5-, 2,6-, 3,4-, and 3,5-dimethylpyridine, 2-ethylpyridine,2,4,6-trimethylpyridine, 4,6-dimethylpyrimidinamine, methoxypyridine,imidazole, 2,4-dimethylimidazol, quinoline, naphtylamine,N,N-dimethylcyclohexylamine, N-ethyldiisopropylamine andhexamethylenetetramine (hexamine). It is preferred that the base usedshould not be prone to separate from the propellant, such as byevaporation.

The base is added in an amount of 0.1 to 5% of the total weight of theother components of the monopropellant, more preferably 0.5–3%, and mostpreferably in an amount of about 0.5–1%.

The presently most preferred bases is hexamine and urea, of which thelatter is more preferred.

Especially preferred fuels are represented by methanol, ethanol,acetone, glycine, and glycerol, of which methanol and glycerol are morepreferred. Methanol is the most preferred fuel in the stabilisedpropellant.

More specifically, a preferred propellant composition according theinvention containing water, and fuel, exhibits a composition rangingfrom 15 to 55% by weight of the fuel in solvent mixture (solventmixture=water+fuel), and a more preferred composition from 10 to 50% byweight of fuel in solvent mixture, and even more preferably, 25 to 45%by weight of fuel in solvent mixture, to which a stabiliser is added inan amount of 0.1 to 5, and more typically 0.5–3%, preferably 0.5–1%, andmost preferably 0.5%, based upon the total weight of the othercomponents.

Accordingly, the most preferred monopropellants are stabilisedcompositions of ADN, water, and glycerol, or ADN, water, and methanol.

In the case of methanol, a composition consisting of about 64.3%ammonium dinitramide, about 24.3% water and about 11.4% by weight ofmethanol (also referred to as LMP-103), to which a stabiliser is addedin the above mentioned amount, is especially preferred.

In the case of glycerol, a composition of about 61.0% of ADN, about26.1% of water, and about 12.9% by weight of glycerol (also referred toas LMP101), to which composition a stabiliser is added in the abovementioned amount, is especially preferred.

The above methanol composition is the most preferred

In order to prolong the catalyst life time, pure propellants should beused so that poisoning of the catalyst is avoided. It is thereforepreferred that the monopropelllant, and notably the AND used should beof highest possible purity. For example, contaminants in the form ofnon-volatile residues, such as for examples iron, potassium andchlorine, should be kept low, such as no more than 50 ppm by weight,more preferably no more than 1 ppm by weight.

Since many of the catalytic metals used according to the presentinvention are known to be suitable catalysts also for other propellants,such as, for example, propellants based on hydroxylammonium nitrate(HAN), the reactor of the invention could also conveniently be used forthe controlled decomposition of such propellants.

While the invention has been described above with reference to specificembodiments thereof, it is apparent that many changes, modifications andvariations can be made without departing from the inventive conceptdisclosed herein. Accordingly, it is intended to embrace all suchchanges, modifications and variations that fall within the spirit andbroad scope of the appended claims.

1. A reactor for decomposition of a liquid ammonium dinitramide-basedmonopropellant into hot, combustible gases, comprising a hollow body (5)provided with, from the upstream end; an injector (20); a heat bed (25);and a catalyst bed (30) of porous catalyst pellets (35) which are heatresistant up to a temperature of at least 1000° C., wherein the injectoris formed so as to be able to distribute the liquid propellant over theheat bed, the overall void volume is essentially formed of the porosityof the heat bed and catalyst bed, and wherein the hollow body isthermally conductive and the heat bed is in indirect thermal contactwith the catalyst bed via the hollow body.
 2. The reactor of claim 1,wherein the downstream portion of the catalyst bed is sinteringresistant up to a higher temperature.
 3. The reactor of claim 2, whereinthe downstream portion of the catalyst bed is sintering resistant up to1200° C.
 4. The reactor of claim 2, wherein the downstream portion ofthe catalyst bed is sintering resistant up to 1400° C.
 5. The reactor ofclaim 2, wherein the downstream portion of the catalyst bed is sinteringresistant up to 1700° C.
 6. The reactor of claim 1, wherein the injectorhas the properties of a heat bed.
 7. The reactor of claim 1, wherein thepellets comprise an alumina material.
 8. The reactor of claim 7, whereinthe alumina material is hexaaluminate AA₁₁O₁₈, wherein A is an alkalineearth or rare earth metal.
 9. Reactor of claim 8, wherein the AA1 ₁₁O₁₈,is obtained by adding a solution of an aluminium alkoxide to awater-in-oil microemulsion, the aqueous phase of which comprises asolution of a water soluble salt of A, whereafter the powder formed isrecovered and calcined.
 10. The reactor of claim 8, wherein A is La. 11.Reactor of claim 1, wherein the material comprising the pellets containsthe dopant Mn.
 12. Reactor of claim 1, wherein the pellets areimpregnated with a catalytically active metal component selected fromPt, Ru, Pd, Pt/Rh, Ir, Rh, Mn or Ir/Rh.
 13. Reactor of claim 1, whereinthe pellets are prepared from a slurry comprising the catalyst material,a solvent, and any desired additives, by means of a drop-generatingorifice to which said slurry is fed, from which orifice the drops arereleased by means of a relative flow of a liquid medium, and formed intospherical bodies in said liquid medium by means of the action of surfacetension, and thereafter treated for consolidation.
 14. A rocket enginefor ammonium dinitramide-based liquid monopropellant, comprising areactor for decomposition of a liquid ammonium dinitramide-basedmonopropellant into hot, combustible gases, comprising a hollow body (5)provided with, from the upstream end; an injector (20); a heat bed (25);and a catalyst bed (30) of partial catalyst pellets (35) which are heatresistant up to a temperature of at least 1000° C., wherein the injectoris formed so as to be able to distribute the liquid propellant over theheat bed, the overall void volume is essentially formed of the porosityof the heat bed and catalyst bed, and wherein the hollow body isthermally conductive and the heat bed is in indirect thermal contactwith the catalyst bed via the hollow body; and a combustion chamber (50)having a combustion chamber void volume, wherein the total void volumeof said rocket engine is essentially formed of the overall void volumeof the porosity of the heat bed and catalyst bed, and the combustionchamber void volume.
 15. The rocket engine of claim 14, wherein theporous catalytic material has been dimensionally pre-stabilised atexpected operational temperature.
 16. The rocket engine of claim 14,wherein the size of the pellets is about one tenth of the inner diameterof the hollow body (5).
 17. The rocket engine of claim 14, wherein thecombustion chamber (50) is lined with iridium (60).
 18. Rocket engine ofclaim 14 having a thrust of 0.5 N to 1 kN.
 19. The rocket engine ofclaim 14, wherein the porous catalytic material has been dimensionallypre-stabilised at 50° above the expected operational temperature. 20.The rocket engine of claim 14 having a thrust of 0.5 N to 50 N.
 21. Aprocess for decomposition of a liquid ammonium dinitramide-basedmonopropellant, comprising the steps of: (A) subjecting a liquidammonium dinitramide-based monopropellant to a temperature efficient foressentially bringing the propellant into the vapour phase anddecomposing the dissolved ammonium dinitramide into gaseous compound;(B) bringing the essentially vaporised monopropellant into contact witha porous catalytic material for decomposition of the monopropellant intohot, gaseous combustible components; and, optionally (C) combusting thecombustible components; and wherein heat generated from step (B) and/or(C) is used for vaporising the liquid monopropellant in step (A). 22.Process of claim 21, wherein the vaporisation of the monopropellant instep (A) is sufficient for preventing vapour induced disintegration ofthe porous catalytic material.
 23. Process of claim 21, wherein astabilised liquid dinitramide-based monopropellant is decomposed.